Knitted heat shrink tubing and method for the production thereof

ABSTRACT

The invention relates to a protective tubing produced using a heat-shrinkable yarn in the circular knitting method, and to a method for producing a protective tubing of this kind.

The invention relates to a protective tubing produced using a heat-shrinkable yarn.

It should already be mentioned at this point that the material property “heat-shrinkable” does not relate to the undesired shrinkage which is known from the domestic field and which occurs, for example, when a garment produced from plastics fibres is washed at too high a temperature. Rather, said material property relates to the desired shrinkage known to a person skilled in the art from the field of heat shrink tubing in the manner in which said tubing is used, for example, for insulating electrical cables. In order to be able to provide this kind of shrinkability, the plastics material must be warmed and expanded in the warmed state. By subsequently cooling the plastics material to below the glass transition temperature thereof, said material is frozen, as it were, in the unstretched state with respect to compression. When warmed again to a temperature above the glass transition temperature, the frozen tensions are released and the material contracts again basically to the initial state thereof.

Protective tubing of the type mentioned at the outset is used, for example, in the construction of motor vehicles, in particular to protect fluid lines, for example charge air lines, brake lines, servo lines, coolant lines or the like, as well as vacuum lines and electrical cables, from external mechanical influences acting thereon.

A protective tubing produced from woven fabric is known from the generic EP 0 906 536 B1. In the known fabric tubing, the peripherally extending threads are produced from a heat-shrinkable material, whereas the threads extending in the axial direction are produced from a material which is not heat-shrinkable. This gives the known fabric tubing the desired anisotropic shrinkage. Although shrinkage is desired in the peripheral direction in order for the tubing, when shrunk, to come into close contact with the line to be protected, in the axial direction at most significantly less shrinkage is acceptable, since said shrinkage makes it more difficult to cut the non-shrunk protective tubing to the correct length.

In addition, the longitudinal seam thereof is disadvantageous in the known fabric tubing, which seam is provided in order to facilitate the production of the fabric tubing. Specifically, since circular looms have an extremely complex construction and are expensive to purchase, fabric heat shrink tubing is in practice usually produced as an elongate fabric band, the two longitudinal edges of which are subsequently laid over each other and interconnected, for example by means of gluing, welding or the like, in order to form tubing. The seam formed thereby is disadvantageous in that it impedes the flexibility of the tubing. Moreover, fabric tubing is known in which two fabric bands are laid over each other and interconnected by means of two seams. These seams also impede the flexibility of the tubing.

The object of the present invention is therefore that of providing a heat shrink protective tubing having anisotropic shrinkage properties, which is produced in a simple and cost-effective manner and so as to be seam-free.

This object is achieved according to the invention by a protective tubing of the type mentioned at the outset, produced using a circular knitting method. The circular knitting method has the advantage that just one single thread or just one single thread bundle need be supplied to the circular knitting machine in order to produce the round-knitted protective tubing. Said method can thus be carried out in a simple manner. Surprisingly, despite the fact that the knitting yarn has the same shrinkage properties over the entire thread length thereof and should therefore contract equally strongly in the knitted fabric, both in the peripherally extending thread portions and in the thread portions extending in the axial direction, anisotropic shrinkage of the knitted tubing occurs. During shrinkage, said tubing contracts significantly more strongly in the peripheral direction than in the longitudinal direction. It is particularly surprising here that the knitted tubing contracts more strongly in the peripheral direction than corresponds to the nominal shrinkability of the thread used during knitting. This point will be discussed in more detail below.

The shrinkability upon heating may be an inherent property of the material from which the yarn is produced. For example, the yarn or at least the components thereof which are heat-shrinkable may be produced from heat-shrinkable polyester, shrinkable polyamide or shrinkable polyethylene.

Moreover, the yarn may comprise at least one monofilament and at least one multifilament. In this case, the at least one multifilament can improve the opacity of the knitted fabric, in particular in the shrunk state thereof, whereas the at least one monofilament can improve the protective function of the knitted fabric. Moreover, not all the filaments of the yarn need have the property of being heat-shrinkable. Rather, it may be sufficient for at least one monofilament and/or at least one multifilament and/or at least a portion of the filaments of at least one multifilament to have this property.

According to an embodiment, the yarn may comprise two monofilaments and one multifilament, which are all produced from heat-shrinkable polyester. In this case, the two monofilaments may have a diameter of approximately 0.18 mm for example, while the multifilament may be 280 dtex. Moreover, the multifilament may comprise at least 50 individual filaments.

Since the diameter of the filament is usually specified for monofilaments whereas the dtex value is usually specified for multifilaments (this value is the weight in grams of a multifilament having a length of 10,000 m), information is also required about the specific weight of the material from which the multifilament is produced, in order to be able to calculate an “effective thread diameter” of the yarn. This parameter, which is used in connection with the present invention in order to characterise the knitting yarn, is calculated in the following manner: an effective cross-sectional area of the multifilament or the multifilaments is calculated from the dtex value of the multifilament or the multifilaments, taking account of the specific weight of the filament material. Then, the values for the cross-sectional areas of all the monofilaments and multifilaments of the yarn are added together, and subsequently the diameter of a circular surface is calculated, the area of which circular surface is equal to the cumulative value. The diameter thus calculated is the “effective thread diameter” of the yarn. It is clear that this value is a hypothetical value, since there may be yarns of which the filaments cannot be arranged so as to be disc-shaped in cross section. For example, an effective thread diameter of 0.30 mm results for a yarn produced from polyester and comprising two monofilaments each having a diameter of 0.18 mm and one multifilament of 280 dtex, taking account of a specific density of polyester of 1.38 g/cm³. In this case, it is easy to see that the two monofilaments arranged beside each other have an extension of 0.36 mm and therefore a circular surface having a diameter of 0.30 mm cannot be achieved. Nonetheless, it has been found that the effective thread diameter, defined as set out above, is a meaningful parameter for describing the knitting yarn, since the yarn tends, in any case, to flatten out at the bonding points of the knitted fabric on account of the interaction of the two thread portions adjoining each other there. As a further example, an effective thread diameter of 0.19 mm results for a yarn produced from polyester and comprising one monofilament having a diameter of 0.18 mm and one multifilament of 280 dtex, taking account of a specific density of polyester of 1.38 g/cm³.

In general, it is preferred for the effective diameter to be between approximately 0.15 mm and approximately 0.40 mm.

In order to be able to increase the amount of desired shrinkage in the peripheral direction, a development of the invention proposes that, in the non-shrunk state of the knitted fabric, the ratio of the length of the stitch period in the peripheral direction of the knitted fabric to the difference between the outer width of the narrower stitches, measured in the peripheral direction, minus the effective thread diameter, be of between approximately 1.49 and approximately 1.78, preferably between approximately 1.54 and approximately 1.70. In the case of lower values, shrinkage in the peripheral direction is impeded by the lateral contact of neighbouring stitches of one stitch row, whereas in the case of larger values the knitted fabric becomes unstable, with the result that the stitches stretch in length and the spacing of the stitch rows in the longitudinal direction of the knitted tubing increases.

Moreover, in order to reduce the undesired shrinkage in the longitudinal direction of the knitted tubing, the ratio of the length of the stitch period in the longitudinal direction of the knitted fabric to the difference between the outer width of the narrower stitches, measured in the peripheral direction, minus the effective thread diameter in the non-shrunk state of the knitted fabric, may be of between approximately 0.65 and approximately 0.76. As a result, the stitch rows follow one another so closely in the longitudinal direction of the knitted tubing that the threads of the preceding and the following stitch rows bound by the stitches of a stitch row in question are either already in contact with one another in the non-shrunk state of the knitted tubing or come into contact with one another at least during shrinkage, and thus at least impede, if not completely prevent, shrinkage or further shrinkage in the longitudinal direction.

In addition or alternatively, shrinkage of the knitted tubing can also be impeded in the longitudinal direction in that the knitted fabric is coated by means of a varnish, preferably a varnish using water as the solvent. The varnish increases the interaction, at the four bonding points of each stitch, between the threads in contact with one another and thus prevents relative movement of said threads as a result of the shrinkage.

In addition or alternatively, shrinkage of the knitted tubing can be further impeded in the longitudinal direction in that a strip of a material which is not heat-shrinkable is arranged on the outer surface and/or the inner surface of said knitted tubing at at least one point, preferably at least two for example diametrically opposed points, of the periphery thereof. The strip may be designed as a thread or a band for example and may be produced from silicone for example.

It should be noted that, in the context of the explanation of the present invention, all the values given for measurement parameters for the stitches of the knitted fabric are to be understood as average values of a plurality of stitches.

According to a further aspect, the present invention relates to a method for producing a protective tubing according to the invention, using a circular knitting machine.

Here, the ratio of the needle width of the needles of the circular knitting machine to the needle spacing, measured from needle centre to needle centre, of the needles may be of between approximately 0.31 and approximately 0.37, preferably between approximately 0.33 and approximately 0.34.

Finally, it has been found to be advantageous for the diameter of the circle of needles of the circular knitting machine to be between approximately one quarter and approximately half, preferably approximately one third, larger than the diameter of the protective tubing in a state in which said tubing is removed from the circular knitting machine and is not shrunk.

It should also be added that a protective tubing formed as knitted tubing is known from EP 0 499 089 B1. In order to ensure close contact between the knitted tubing and the line to be protected, this document proposes, however, producing the knitted fabric from a rubber-resilient material or at least embedding rubber-resilient material in the yarn.

The invention will be described in more detail in the following, on the basis of an embodiment and with reference to the accompanying drawings, in which:

FIG. 1 is an enlarged detail of a portion of the knitted fabric; and

FIG. 2 is a schematic perspective view of the knitted tubing according to the invention.

FIG. 1 shows a knitted fabric 10 produced on a circular knitting machine. The yarn 12 used for producing the knitted fabric 10 comprised two polyester monofilaments each having a diameter of 0.18 mm and one polyester multifilament of 280 dtex. Taking account of a specific weight of polyester of 1.38 g/cm³, this results in an effective thread diameter of 0.30 cm.

This yarn was knitted into knitted tubing 20 (see the highly schematic view in FIG. 2) using a single jersey circular knitting machine. The needles of the circular knitting machine had a width of 0.90 mm and a needle spacing, measured from needle centre to needle centre, of 2.65 mm. After being removed from the circular knitting machine, the knitted tubing had a diameter of approximately 30 mm in the non-shrunk state. The stitch rows M of the knitted fabric followed one another at a period p of approximately 0.9 mm in the longitudinal direction L. The stitch period P in the peripheral direction U was approximately 1.8 mm, and the spacing D of the narrow stitches m adjacent to one another in the peripheral direction was approximately 0.27 mm.

Regarding the term “narrow stitch” it should be noted that, by rotating FIG. 1 by 180°, it is possible to also view the knitted fabric 10 as being formed of wide stitches n arranged in close succession, the loop head of which wide stitches is formed by the thread portion forming the spacing D of the narrow stitches m.

As shown by a dashed line in FIG. 1, the thread portions F₁ of the preceding stitch row M₀ and F₂ of the following stitch row M₂ bound by a narrow stitch m of a stitch row M₁ in question are already in contact with one another in the knitted fabric 10 in the non-shrunk state. When the knitted fabric 10 is heated, said thread portions thus resist shrinkage in the longitudinal direction L, whereas shrinkage is easily possible in the peripheral direction U on account of the free lengths of thread between adjacent stitches. The knitted tubing 20 can therefore shrink in the peripheral direction U to a diameter of approximately 20 mm by means of heating.

The protective tubing 20 described above was produced on a circular knitting machine comprising 48 needles, each having a needle width of approximately 0.9 mm and a needle spacing, measured from needle centre to needle centre, of approximately 2.65 mm.

As indicated in FIG. 2, a strip 22 of a material which is not heat-shrinkable can be respectively arranged at at least two points, which are preferably diametrically opposed, on the outer peripheral surface 20 a of the knitted tubing 20 in order to impede shrinkage of the knitted tubing 20 in the longitudinal direction L. Similar shrinkage-impeding strips may also be arranged on the inner surface 20 b of the knitted tubing 20. 

1. Protective tubing produced using a heat-shrinkable yarn, wherein the protective tubing is produced using a circular knitting method.
 2. The protective tubing according to claim 1, wherein, in a non-shrunk state of a knitted fabric, the ratio (P/M−Ø_(eff)) of the length of a stitch period (P) in the peripheral direction of the knitted fabric to the difference (M−Ø_(eff)) between the outer width (M) of narrower stitches, measured in the peripheral direction, minus the effective thread diameter (Ø_(eff)), is between approximately 1.49 and approximately 1.78.
 3. The protective tubing according to claim 1, wherein, in the non-shrunk state of the knitted fabric, the ratio (p/M−Ø_(eff)) of the length of the stitch period (P) in the longitudinal direction of the knitted fabric to the difference (M−Ø_(eff)) between the outer width (M) of the narrower stitches, measured in the peripheral direction, minus the effective thread diameter (Ø_(eff)), is between approximately 0.65 and approximately 0.76.
 4. The protective tubing according to claim 1, wherein the yarn has an effective thread diameter (Ø_(eff)) of between approximately 0.15 mm and approximately 0.40 mm.
 5. The protective tubing according to claim 1, wherein the yarn comprises at least one monofilament and at least one multifilament.
 6. The protective tubing according to claim 5, wherein at least one monofilament and/or at least one multifilament and/or at least a portion of the filaments of at least one multifilament is produced from heat-shrinkable polyester.
 7. The protective tubing according to claim 1, wherein the knitted fabric is coated by a varnish.
 8. The protective tubing according to claim 1, wherein a strip of a material which is not heat-shrinkable is arranged on an outer surface and/or an inner surface thereof at at least one point.
 9. A method for producing a protective tubing according to claim 1, wherein the protective tubing is produced on a circular knitting machine.
 10. The method according to claim 9, wherein the ratio of the needle width of needles of the circular knitting machine to the needle spacing, measured from needle center to needle center, of the needles is between approximately 0.31 and approximately 0.37.
 11. The according to claim 9, wherein the diameter of a circle of needles of the circular knitting machine is between approximately one quarter and approximately half larger than the diameter of the protective tubing in a state in which said tubing is removed from the circular knitting machine and is not shrunk. 